As an alternative to the viscose process, in recent years there has been described a number of processes in which cellulose, without derivatization, is dissolved in an organic solvent, a combination of an organic solvent and an inorganic salt, or in aqueous salt solutions. Cellulose fibres made from such solutions have received by BISFA (The International Bureau for the Standardisation of man made Fibres) the generic name Lyocell. As Lyocell, BISFA defines a cellulose fibre obtained by a spinning process from an organic solvent. By "organic solvent", BISFA understands a mixture of an organic chemical and water. "Solvent-spinning" is considered to mean dissolving and spinning without derivatization.
So far, however, only one process for the production of a cellulose fibre of the Lyocell type has achieved industrial-scale realization. In this process, N-methylmorpholine-N-oxide (NMMO) is used as a solvent. For the purposes of the present specification, the abbreviation "NMMO" will be used instead of the expression "tertiary amine-oxides", NMMO denoting additionally N-methylmorpholine-N-oxide, which today is preferably used.
Tertiary amine-oxides have been known for a long time as alternative solvents for cellulose. Thus it is known for instance from U.S. Pat. No. 2,179,181 that tertiary amine-oxides are capable of dissolving high-grade chemical pulp without derivatisation and that from these solutions cellulose moulded bodies, such as fibres, may be obtained by precipitation. In U.S. Pat. Nos. 3,447,939, 3,447,956 and 3,508,941, further processes for the production of cellulose solutions, wherein cyclic amine-oxides are preferably used as solvents, are described. In all these processes, cellulose is dissolved physically at elevated temperature.
When the solution is prepared in a double screw extruder or in a stirring vessel, the pulp has to be subjected to a preactivation in order to carry out the dissolution process at sufficient speed (see "Das Papier", edition 12, pages 784-788). As preactivation, the formation and regeneration of alkali cellulose or a hydrothermical processing of the pulp have been proposed.
According to DD-A - 226 573, before the preparation of a solution, which also is carried out in an extruder, the cellulose is preactivated as well. Said DD-A starts from a suspension of cellulose containing NMMO, which first is homogenized in a stirring vessel. Then the substance density is increased to 12.5% by mass by means of centrifuging or squeezing out, whereafter the suspension is dried to a water content of from 10-15% by mass (based on NMMO) and converted to a clear solution in an extruder provided with a degassing zone at temperatures of from 75.degree. to 120.degree. C.
In EP-A - 0 356 419 of the applicant, a process carried out in a so-called Filmtruder is described, wherein a suspension of the shredded high-grade pulp in an aqueous tertiary amine-oxide is transported, spread as a thin layer, along a heated surface, the surface of this thin layer being subjected to reduced pressure. As the suspension is transported along the heated surface, water is evaporated and the cellulose can be dissolved, so that a spinnable cellulose solution can be removed from the Filmtruder.
All of the processes described above use high-grade chemical pulp, which is obtained e.g. from beech or spruce wood, as starting material. There is little known in the art about use of alternative cellulose-containing materials.
A processing of lignocellulose materials in the NMMO process is known from WO 86/05526. For this processing, relatively aggressive conditions are recommended. Thus for instance poplar wood is first subjected to a special hydrolysis process and the solid product thereby obtained mixed at room temperature with NMMO having a water content of 13,5%. The NMMO used is the monohydrate of NMMO, present in solid state at room temperature (melting point&gt;70.degree. C.). The solid mixture is homogenized, heated to 130.degree. C. and melt, the hydrolysed wood dissolving.
In "Holzforschung", 42, pages 21-27 (1988) it is also described that lignocellulose material may be dissolved in a solution of NMMO in dimethylsulphoxide. The NMMO used is not an aqueous solution, but has a water content of 15%, also corresponding approximately to the monohydrate.
It is desirable to employ less aggressive conditions for the production of cellulose solutions or to avoid a melting process completely, since from literature it is known that cellulose as well as NMMO are subjected to a degradation process at elevated temperature, the degradation products deteriorating the physical parameters of the Lyocell fibres, such as strength and elongation.
It is the object of the invention to provide a process for the production of cellulose moulded bodies, particularly Lyocell fibres, using alternative pulp materials, i.e. no cellulose from coniferous and deciduous trees, and carrying out the preparation of the solution in the least possible aggressive way. It is desired to avoid melting of a solid mixture product.